This invention relates to a method for determining defective layers of carbon on reactor fuel particles, and more particularly for determining any defective final layers of carbon on triso-coated fuel particles, such as to provide substantially crack-free fuel particles for making fuel rods to thus assure the more efficient operation of such fuel rods in a reactor in which they are to be utilized. The invention described herein is a result of a contract with the U.S. Department of Energy.
Fuel rods for use in a high temperature gas-cooled reactor (HTGR) are right circular cylinders of a carbonized matrix containing fissile, fertile and inert particles to provide a given composition for a particular location in the reactor. The particles are normally small spheres; the fissile particles being carbon-coated microspheres of a uranium oxicarbide, and the fertile particles being carbon-coated microspheres of thorium oxide. The fissile particles are designated as triso-coated particles, which coatings include an inner porous carbon coating, an inner low-temperature isotropic (LTI) carbon coating over the porous layer, a silicon carbide coating over the inner LTI layer, and a final, outer LTI carbon coating.
During the fabrication of such particles, samples are inspected after each step to assure high integrity for each layer in order to determine that undamaged coated particles are utilized in the making of fuel rods. Other tests are conducted relative to size, shape, density, etc.
A conventional test for the integrity of the inner LTI layer involves subjecting the particles to a flowing stream of chlorine gas at 1500.degree. C. If the coating has a flaw, the chlorine reacts with the uranium in the kernel of the particle to produce uranium chloride which is then detected in the outlet gas stream. The degree of porosity of such coating can be detected using gas absorption techniques or by mercury porosimetry.
A different method is utilized to study the integrity of the SiC layer after deposition thereof on the inner LTI layer. The SiC-coated particle is subjected to pressurized mercury and thereafter is radiographed. If a crack exists in the SiC coating, the mercury will be observed within the coating and between the layers of the SiC and the inner LTI coating.
Prior to the present invention, no satisfactory method existed to test the integrity of the outer LTI layer. As above, the degree of porosity can be measured by gas absorption techniques; however, this method does not distinguish between some general surface porosity and a fine crack that extends through the layer to the SiC coating.
It is thus an object of the present invention to provide a method for determining the existence of cracks penetrating the outer layer of an isotropic vapor-deposited carbon coating covering the silicon carbide coating of a triso-coated fuel particle(s).
Other objects, advantages and novel features of the present invention will become apparent to those skilled in the art upon examination of the following detailed description of a preferred embodiment of the invention and the accompanying drawings.